Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
Fuel cells generally include three components: a cathode, an anode and an electrolyte which is sandwiched between the cathode and the anode and passes only protons. Each electrode is coated on one side by a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series electrically to generate increasingly larger quantities of electricity.
In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer electrode membrane serves as the electrolyte between a cathode and an anode. The polymer electrode membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate proton conductivity. Therefore, maintaining the proper level of humidity in the membrane, through humidity/water management, is desirable for the proper functioning of the fuel cell. Irreversible damage to the fuel cell may occur if the membrane dries out.
In order to prevent leakage of the hydrogen fuel gas and oxygen gas supplied to the electrodes and prevent mixing of the gases, a gas-sealing material and gaskets are arranged on the periphery of the electrodes, with the polymer electrolyte membrane sandwiched there between. The sealing material and gaskets are assembled into a single part together with the electrodes and polymer electrolyte membrane to form a membrane and electrode assembly or membrane electrode assembly (MEA). Disposed outside of the MEA are conductive separator plates (unipolar or bipolar plates) for mechanically securing the MEA and electrically connecting adjacent MEAs in series. A portion of the separator plate, which is disposed in contact with the MEA, is provided with a gas passage for supplying hydrogen or oxygen fuel gas to the electrode surface and removing generated water vapor.
Multi-cell PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series and separated one from the next by a gas-impermeable, electrically-conductive separator plate or a bipolar plate. Such multi-cell fuel cells are known as fuel cell stacks. The bipolar plate has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and electrically conducts current between the adjacent cells. Current collectors at the ends of the stack contact the end cells. The separator plate contains a flow field that distributes the gaseous reactants (e.g. H2 and O2/air) over the surfaces of the anode and the cathode. These flow fields generally include a plurality of lands which define a plurality of flow channels through which the gaseous reactants flow between a supply header and an exhaust header located at opposite ends of the flow channels.
The fuel cell stack is usually held together in its assembled state by side plates and end plates. Fuel and air inlet manifolds are connected to a stack inlet port. The stack inlet port directs fuel and air to its respective manifold that distributes the fuel and air to the surfaces of the anode and cathode respectively. Additionally, the stack typically includes a manifold connected to the inlet port for directing coolant to interior channels within the cells of the stack to absorb the heat generated in the cells. The stack also includes an outlet port connected to outlet manifolds for exhausting unreacted fuel and air, each of which carries entrained water, as well as an outlet manifold for the coolant liquid exiting the stack.
A highly porous (i.e. ca. 60%-80%), electrically-conductive material (e.g. cloth, screen, paper, foam, etc.) known as “diffusion media” or gas diffusion media (GDM) is interposed between the current collectors and the MEA and serves (1) to distribute gaseous reactant over the entire face of the electrode, between and under the lands of the current collector, and (2) collects current from the face of the electrode confronting a groove, and conveys it to the adjacent lands that define that groove.
Because the proton conductivity of PEM fuel cell membranes deteriorates rapidly as the membranes dry out, external humidification is required to maintain hydration of the membranes and sustain proper fuel cell functioning. Often, water is added to the anode feed stream. Moreover, the presence of liquid water in automotive fuel cells is unavoidable because appreciable quantities of water are generated as a by-product of the electrochemical reactions during fuel cell operation. Furthermore, saturation of the fuel cell membranes with water can result from rapid changes in temperature, relative humidity, and operating and shutdown conditions. Excessive membrane hydration may result in flooding, excessive swelling of the membranes, degradation of cell performance, and the formation of differential pressure gradients across the fuel cell stack.
Because the balance of water in a fuel cell is important to operation of the fuel cell, water management has a major impact on the performance and durability of fuel cells. Fuel cell degradation can occur with excessive water. Long-term exposure of the membrane to water can also cause irreversible material degradation. Cells closest to the inlets and outlets are particularly susceptible to degradation due to excessive water. More specifically, when water enters the fuel cell stack through the reactant inlets, more water tends to enter the cells closest to the stack inlet causing a relatively lower and unstable voltage in those cells. It is, therefore, desirable to provide a fuel cell stack that reduces the effect on cell performance of water entering the fuel cell stack.